An update on my outside research:
I messaged Premier yesterday to ask them about whether I could use their heating plate in ambient temperatures in the 20s and 30s and the representative said that they recommend a warmer room temperature, and if it's too cool, the chicks won't spend enough time out eating and drinking. I know that folks have indeed had success in some lower temps with the Premier (like you,
@henless), but I think I'm too nervous to risk it, in case my particular situation is set up in a way that won't be as successful. I really appreciate you sharing your experience, though! It helps us all learn more about the options. I'm also grateful to Premier for being upfront about their recommendations. I know that their heating plates are very popular and I think I'd definitely look more seriously at it if I was brooding in a warmer environment.
I also messaged Sweeter Heater, because after visiting their own website, I was unsure that I'd selected the right size for my 15 chicks and that perhaps I'd need a larger one for colder temps. This was from their reply, "The OH11x16 is plenty big for 15 chicks. The larger size has more surface room, but is not any hotter than the other sizes. However, in temps that cold, you will want to make a cave out of the heater. If you take a towel or blanket, and drape it over the top (the top is cold to the touch), and leave one side open for them to come and go, you’ll trap more heat underneath. For new chicks, you’ll want it right above their heads, about 4-6 inches from the grounds. You’ll want to watch them – if they pile, they are cold. If they an panting and laying outside of the edges, they are hot and you’ll want to raise it up."
I also found through the Sweeter Heater website that Gail Damerow (who wrote Storey's Guide to Raising Chickens and is a well-known chicken expert) has used their heaters in some of their brooders and has
written in Backyard Poultry Magazine about her set-up using them. Sweeter Heater also says on their website that they've been making heaters since 1995 and haven't been involved with a fire issue.
So now I'm feeling much better about giving the Sweeter Heater a try. I am wondering if I could test the temperature under it using a thermometer in a container of water? The Sweeter Heater is designed to heat solid objects rather than the air, so I'd think that water would heat up, right? Maybe that's a silly idea.

I just want to start experimenting with it and figuring out my system before my chicks arrive at the end of April. With standard heat bulbs it's simple to just stick a thermometer underneath and monitor the temperature fluctuations. Radiant heat will be a new ballgame.